J. T. Yates

Extraction of kinetic parameters in temperature programmed desorption: A comparison of methods

An investigation of the temperature programmed desorption (TPD) of CO and D2 from Ni(111) has been carried out. It has been shown that a differential method for the extraction of the kinetic parameters, threshold temperature programmed desorption (TTPD), can be applied with accuracy near the limit of zero coverage. In this limit, agreement is found between integral and differential methods for kinetic parameter evaluation. The factors which limit the applicability of TTPD are explored and a method to verify its proper application is presented.

Physical adsorption of xenon in open single walled carbon nanotubes: Observation of a quasi-one-dimensional confined Xe phase

The adsorption of Xe into carbon single walled nanotubes with both closed and open ends has been investigated using temperature programmed desorption and other surface analytical tools. It has been found that opening the ends of the nanotube by chemical cutting increases both the kinetic rate and the saturation capacity of the nanotubes for Xe at 95 K. Further enhancement in Xe adsorption kinetics and capacity are achieved by treating the nanotubes in vacuum at 1073 K where CO, CO2, CH4, and H2 are evolved. On this basis it is postulated that surface functionalities such as −COOH block entry ports for adsorption at the nanotube ends and at the defect sites on the walls. The thermal destruction of these functionalities leads to enhanced adsorption. The denser phase of Xe inside the saturated nanotubes desorbs by zero-order kinetics (Ed=26.8±0.6 kJ/mol). It is postulated that a quasi-one-dimensional Xe confined phase in equilibrium, with a two-dimensional Xe gas phase on the exterior, provides a phase trans...

Adsorption and thermal behavior of ethylene on Si(100)-(2 × 1)

The adsorption of ethylene on Si(100)-(2 × 1) has been studied in ultrahigh vacuum. Chemisorption was found to occur via a mobile precursor mechanism. The activation energy difference for desorption and chemisorption from the precursor (Ed − Er), was found to be 2.9 kcal mol−1. The saturation coverage of ethylene is 1 C2H4/Si2 dimer. Hydrogen-site blocking and thermochemical arguments suggest that C2H4 bonds as a di-σ surface complex to dimer sites; upon chemisorption of C2H4 the SiSi dimer is cleaved. Chemisorbed ethylene desorbs unimolecularlfrom Si(100) at ∼ 550 K, with approximately 2% of the monolayer undergoing dissociation. The activation energy of C2H4 desorption is 38 kcal mol−1, and for the di-σ C2H4Si2 complex, each SiC bond has a strength of ∼ 73 kcal mol−1. The low desorption activation energy allows C2H4 to desorb prior to signifi dissociation, preventing the formation of significant coverages of surfaces carbon and hydrogen.

Kinetics and energetics of oxygen adsorption on Pt(111) and Pt(112)- A comparison of flat and stepped surfaces

A comparative study of oxygen adsorption on Pt(111) and Pt(112) has been performed using temperature programmed desorption, isothermal desorption, Auger spectroscopy, LEED and isotopic measurements. On Pt(112) three molecular adsorption states (α1, α2, α3) and two atomic adsorption states (β1, β2) have been found. The β2-state exhibits repulsive lateral interaction whereas the β1-state shows attractive interaction. The adsorption kinetics at Tad = 87 K involves a precursor state. For Pt(112) at 87 K, the sticking coefficient is 0.97 at zero coverage and remains constant in the low coverage regime. On Pt(111) at 87 K, the sticking coeffient increases with increasing oxygen coverage at low coverage, with s0 = 0.29. This suggests that empty Pt sites near an O2-covered Pt site experience an enhanced reactivity with O2. Tad = 300 K the adsorption kinetics are governed by direct dissociative adsorption with an activation barrier of ≈2 kal/mol on Pt(111), yielding an initial sticking probability of 0.05, whereas a complicated adsorption behavior is obtained for Pt(112) with s0 = 0.53. The conversion of molecular oxygen into atomic oxygen is discussed as well as the influence of subsurface oxygen and “clean-off” effects on the adsorption kinetics.

Methods in semiconductor surface chemistry

Methods for studying semiconductor surface chemistry are presented. It is shown that adsorption and desorption kinetic measurements, when combined with Auger spectroscopy, can give useful insights into fundamental elementary surface kinetic processes which are important in understanding the behavior of complex chemical vapor deposition, plasma vapor deposition, or reactive ion etching processes. Techniques for crystal preparation, mounting, temperature control, and reaction kinetic measurements are given using examples from the adsorption and reaction of propylene with Si(100). An illustration of the manipulation of active site availability on Si(100) is described.

Reaction of methanol with Cu(111) and Cu(111) + O(ads)

Abstract The reactive chemistry of methanol on the Cu(111) surface, both with and without preadsorbed oxygen atoms, is investigated between 190 and 700 K. The clean Cu(111) surface is inert, and molecularly absorbed methanol, the only stable surface species identified on this surface, desorbs at about 210 K. Various trends are examined as a function of oxygen coverage, from the clean surface to the saturation oxygen coverage (approximately 0.45 O atom/Cu atom). The capacity of the surface to adsorb methanol (190 K), and the formaldehyde yield (∼ 400–450 K) are both maximized when the oxygen coverage is about 0.26 O atom/Cu atom. Trends in the yield of other products, and the temperature for decomposition of the stable methoxy intermediate are examined. Also, the rate of methoxy decomposition is limited by CH bond breaking as evidenced by a deuterium kinetic isotope effect (CH versus CD). A minor decomposition path for methanol on O + Cu(111) involves CO2 formation, probably via a formate surface intermediate. Preadsorbed oxygen serves as an acceptor of the methanol hydroxyl hydrogen, enabling facile methanol conversion to methoxy at low oxygen coverage for T ⩾ 190K. However. at high oxygen coverage ( θ ⪆ 0.26 O atom/Cu atom ) oxygen inhibits surface reactivitv. A two-dimensional model which defines three types of surface sites is used to explain the general trend of methanol reactivity as a function of oxygen coverage.

Bond activation sequence observed in the chemisorption and surface reaction of ethanol on Ni(111)

Abstract The mechanism of ethanol decomposition on the Ni(111) surface has been investigated between 155 and 500 K. The sequence of bond scission steps which occur as ethanol undergoes dissociative reactions on this surface has been deduced using deuterium and 13 C isotopic labels. Bond activation occurs in the order (1) OH, (2) CH 2 (methylene CH), (3) CC, (4) CH 3 (methyl CH). The products observed are CH 3 CHO(g), CH 4 (g), CO(g), H 2 (g) and surface carbon, C(a). The latter species exhibits a carbidic AES lineshape in the temperature range 450 to 670 K, at which temperature it dissolves into the Ni bulk. Acetaldehyde, CH 3 CHO, and methane, CH 4 , desorb with the same threshold temperature (260–265 K), and the formation of both of these products is controlled by scission of the methylene CH bond (CH 2 group). The CH 3 group is cleaved from the intermediate surface CH 3 CHO species to form CH 3 (ads). H 2 exhibits a broad, doublet desorption peak from 300 to 450 K. The carbonoxygen bond in ethanol remains intact and CO ultimately desorbs in a single desorption limited process ( T p = 430 K). A small fraction of CO(a) species undergo exchange with the carbidic surface carbon in a minor process observed above 440 K.

Infrared spectroscopic observations of surface bonding in physical adsorption: The physical adsorption of CO on SiO2 surfaces

Abstract The physical adsorption of CO onto SiO2 surfaces containing isolated OH groups results in the formation of two types of physisorbed CO. The first, CO species A, exhibits vCO = 2158 cm−1 and is bound by hydrogen bonding to the SiOHδ+ groups to form a surface complex SiOHδ+ … CO. The strength of this interaction is about 2.7 kcal mol−. The second form of physisorbed CO, CO species B, exhibits vCO = 2140 cm−1, and has pronounced rotational wings. The degree of rotational freedom is determined by the extent of shielding by CO of the polar groups on the surface. At high coverages of physisorbed CO species B, solvent effects due to CO species B on both the OH and CO stretching modes of the SiOH … CO complex are observed to cause small downward shifts in stretching frequency.

Wide temperature range IR spectroscopy cell for studies of adsorption and desorption on high area solids

A new design for an infrared cell useful for studies of the spectrum of surface species on high area solids is presented. The cell is well suited over a wide temperature range (100–1000 K). Other demonstrated features of the cell include ultrahigh‐vacuum operation, temperature control to ±1 K, linear and rapid temperature programmability and low‐temperature gradients across the powdered sample. The method of sample preparation and support minimizes both heat and mass transport effects. A detailed literature search of previous infrared cell designs is included. Results of the application of the new cell design to the high‐temperature dehydroxylation of Al2O3 are given as an example of the performance.

IRAS study of the adsorption of CO on Ni(111): Interrelation between various bonding modes of chemisorbed CO

Abstract The IRAS (infrared reflection absorption spectroscopy) method has been applied to the study of CO adsorption on Ni(111). Evidence has been found for the coexistence of two-fold and three-fold coordinated CO species at low CO coverages. A continuous coverage-dependent conversion from three-fold to two-fold bridged CO has been observed. Accurate correlation of the development of the IRAS spectra with CO coverage, with CO temperature programmed desorption behavior, and with LEED behavior has been established through studies of the frequency and lineshape of the spectral features. Evidence for inhomogeneous CO layers at the highest CO coverages was obtained. This work establishes baseline behavior for other IRAS studies of this system to be reported.